In recent years, there has been strong demand for lighter and thinner displays for personal computers, television sets and the like, and therefore liquid crystal display devices, which readily meet such demand for lighter and thinner displays, have been increasingly employed. However, the liquid crystals are slow in response speed, and therefore in the case of displaying moving images on the liquid crystal display device, it might not be possible to obtain satisfactory image quality. Accordingly, a drive system called “overshoot drive” has been conventionally employed in order to suppress moving images from being displayed in low quality due to the low response speed of the liquid crystals. The overshoot drive is a drive system in which a drive voltage higher or lower than a predetermined gradation voltage that corresponds to an image signal for the current frame is supplied to a liquid crystal display panel in accordance with a combination of an image signal for the immediately preceding frame and the image signal for the current frame. By employing the overshoot drive, a time period required for reaching the predetermined gradation voltage that corresponds to the image signal for the current frame is shortened, so that the moving images on the liquid crystal display device are suppressed from being displayed in low quality.
In the liquid crystal display device employing the overshoot drive, a look-up table as described below is held, such that a drive voltage is determined in accordance with a combination of a gradation value that corresponds to the image signal for the immediately preceding frame (hereinafter, referred to as the “prior gradation value”) and a gradation value that corresponds to the image signal for the current frame (hereinafter, referred to as the “posterior gradation value”). FIG. 11 is a diagram schematically illustrating the contents of a conventional look-up table held in a liquid crystal display device capable of 256-gradation display. In FIG. 11, numerical values shown in the leftmost column indicate prior gradation values, and numerical values shown in the uppermost row indicate posterior gradation values. Furthermore, numerical values shown at intersections between the rows and columns indicate gradation values (hereinafter, referred to as the “applied gradation values”) corresponding to drive voltages, which are determined in accordance with combinations of their respective prior gradation values and posterior gradation values. For example, when the prior gradation value is “64” and the posterior gradation value is “128”, the applied gradation value is “155”. Note that FIG. 11 shows only nine typical examples of 256 gradation values for each of the prior gradation value and the posterior gradation value.
Incidentally, in the case of the 256-gradation display, there are 65,536 (=256×256) combinations of the prior gradation value and the posterior gradation value. Accordingly, 65,536 applied gradation values have to be stored in the look-up table. In other words, in order to configure the look-up table, a memory capacity capable of storing the 65,536 applied gradation values is required. Also, the number of bits required for representing each of the 256 gradations is eight. Therefore, eight bits are required for each of the 65,536 applied gradation values.
In addition, as described above, in the case of performing the overshoot drive, the applied gradation value is determined in accordance with a combination of the prior gradation value and the posterior gradation value with reference to the look-up table. Accordingly, the prior gradation value has to be held for each pixel in a frame. Therefore, the liquid crystal display device employing the overshoot drive is provided with a memory device, such as a RAM (Random Access Memory), which is called the “frame memory”, in order to hold the prior gradation value for each pixel in the frame. For example, in the case of a liquid crystal display device with 480 scanning signal lines and 640 video signal lines, the number of pixels is 307,200 (=480×640). Since the number of bits (bit number) required for representing each of the 256 gradations is eight, eight bits are required for each prior gradation value for the 307,200 pixels.
As such, in order to determine the applied gradation value in accordance with a combination of the prior gradation value and the posterior gradation value, the liquid crystal display device employing the overshoot drive requires the above-described look-up table and frame memory. However, recent years have seen an increase in demand for more compact mobile terminal devices such as cell phones, and in order to realize further compactness, it is necessary to achieve a reduction in required memory capacity.
Therefore, Japanese Laid-Open Patent Publication No. 2004-4629 discloses a liquid crystal display device with reduced memory capacity for the look-up table. FIG. 12 is a diagram schematically illustrating the contents of the look-up table held in the liquid crystal display device. For each of the prior gradation value and the posterior gradation value, only nine of 256 gradation values are stored in the look-up table. As shown in FIG. 12, the applied gradation value is stored in the look-up table in association with each combination of the nine prior gradation values and the nine posterior gradation values. That is, the number of applied gradation-values stored in the look-up table is 81 (=9×9).
In the case, for example, where the prior gradation value is “128”, and the posterior gradation value is “192”, the applied gradation value is determined as “203” according to the look-up table. On the other hand, in the case, for example, where the prior gradation value is “16”, and the posterior gradation value is “80”, the applied gradation value cannot be determined directly from the values stored in the look-up table. In such a case, the applied gradation value is determined by interpolation calculation based on applied gradation values obtained in the cases where: the prior gradation value is “0” and the posterior gradation value is “64”; the prior gradation value is “0” and the posterior gradation value is “96”; the prior gradation value is “32” and the posterior gradation value is “64”; and the prior gradation value is “32” and the posterior gradation value is “96”. In this manner, by calculating the applied gradation value by interpolation calculation, it becomes possible to reduce the memory capacity required for the look-up table.
In addition, Japanese Laid-Open Patent Publication No. 2004-109796 discloses a liquid crystal display device in which the applied gradation value is determined based on the upper four bits of the prior gradation value corresponding to the image signal for the immediately preceding frame stored in the frame memory and the upper four bits of the posterior gradation value corresponding to the image signal for the current frame. According to this liquid crystal display device, the look-up table holds values, each of which indicates a corresponding one of 16 sections into which the 256 gradation values are classified, instead of holding the prior gradation values and the posterior gradation values. In addition, the number of applied gradation values stored in the look-up table is 256.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-4629    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-109796